Polycrystalline diamond (PCD) materials known in the art are formed from diamond grains (or crystals) and a catalyst material which are subjected to high pressure and high temperature conditions (“HPHT sintering process”). Such PCD materials are known for having a high degree of wear resistance, making them a popular material choice for use in such industrial applications as cutting tools for machining and wear and cutting elements in subterranean mining and drilling, where such high degree of wear resistance is desired. In such applications, conventional PCD materials can be provided in the form of a surface layer or body to impart desired levels of wear resistance to a cutting tool.
Traditionally, PCD cutting elements include a substrate and a PCD body or layer attached thereto. Substrates used in such cutting element applications include carbides such as a cemented tungsten carbide (e.g., WC—Co). Such conventional PCD bodies utilize a catalyst material to facilitate intercrystalline bonding between the diamond grains and to bond the PCD body to the underlying substrate. Metals conventionally employed as the catalyst are often selected from the group of solvent metal catalysts including cobalt, iron, nickel, combinations, and alloys thereof.
The amount of catalyst material used to form the PCD body represents a compromise between desired properties of strength/toughness/impact resistance and hardness/wear resistance/thermal stability. While a higher metal catalyst content typically increases the strength, toughness and impact resistance of a resulting PCD body, such higher metal catalyst content also decreases the hardness and corresponding wear resistance as well as the thermal stability of the PCD body. Thus, these inversely affected properties ultimately limit the ability to provide PCD bodies having desired levels of hardness, wear resistance, thermal stability, strength, impact resistance, and toughness to meet the service demands of particular applications, such as cutting and/or wear elements used in subterranean drilling devices.
A particularly desired property of PCD bodies used for certain applications is improved thermal stability during wear or cutting operations. A problem known to exist with conventional PCD bodies is that they are vulnerable to thermal degradation when exposed to elevated temperature cutting and/or wear applications. This vulnerability results from the differential that exists between the thermal expansion characteristics of the solvent metal catalyst material disposed interstitially within the PCD body and the thermal expansion characteristics of the intercrystalline bonded diamond. Such differential thermal expansion is known to start at temperatures as low as 400° C., and can induce thermal stresses that can be detrimental to the intercrystalline bonding of diamond and eventually result in the formation of cracks that can make the PCD structure vulnerable to failure. Accordingly, such behavior is not desirable.
Another form of thermal degradation known to exist with conventional PCD materials is one that is also related to the presence of the solvent metal catalyst in the interstitial regions of the PCD body and the adherence of the solvent metal catalyst to the diamond crystals. Specifically, the solvent metal catalyst is known to cause an undesired catalyzed phase transformation in diamond (converting it to carbon monoxide, carbon dioxide, or graphite) with increasing temperature, thereby limiting practical use of the PCD body to about 750° C.
Thermal degradation can lead to chipping, spalling, partial fracturing, and/or exfoliation of the PCD body. These problems can be caused by the formation of micro-cracks within the PCD body followed by propagation of the crack across the PCD body. Micro-cracks can form from thermal stresses occurring within the PCD body.
U.S. Pat. No. 6,601,662 (“the '662 patent”) relates to cutting elements comprising a PCD body with improved wear resistance and methods of manufacturing such cutting elements. The cutting elements described have a PCD body having a diamond volume density of greater than 85% and contain an interstitial matrix in the PCD body adjacent to a working surface which is substantially free of the catalyzing material. The '662 patent teaches that in order to achieve a sufficient level of wear resistance, increasing the volume density of diamond leads to a reduction in the depth of interstitial matrix which is substantially free of catalyzing material.
U.S. Pat. No. 7,493,973 (“the '973 patent”) relates to cutting elements comprising a PCD body with a high diamond content which is treated to provide improved properties of abrasion resistance and thermal stability, while maintaining a desired degree of impact resistance. The high diamond content is obtained using coarse-sized diamond grains, such as diamond grains having an average particle size of about 0.03 mm or greater. The '973 patent also teaches that the diamond volume content of the region of the PCD body to be treated (rendered substantially free of catalyst material) will impact the depth of treatment needed to obtain a desired level of performance such as wear resistance. In particular, the '973 patent teaches that for a diamond content of greater than about 93% by volume (% v), the average depth of treatment is less than about 0.08 mm (millimeters) (80 microns/micrometers) and for a diamond content of at least about 95% by volume (% v), the average depth of treatment is at most about 0.03 mm (30 microns).
Although much work has been done with respect to the PCD body used to form a cutting element, it is still desirable that a PCD body be developed that displays even greater improvements in properties such as wear resistance and thermal stability while not sacrificing desired strength, toughness or impact resistance, especially for difficult drilling applications. Examples of difficult drilling applications include abrasive sandstones such as those found in the East Texas Basin and geothermal applications.